This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. NUTRITIONAL REGULATION OF CHONDROCYTES ORIGINAL ABSTRACT Linear bone growth and maintenance of articular cartilage both require the proliferation and differentiation of chondrocytes. Much of the work done on chondrogenesis has focused on the role of the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis. While it has long been known that nutrient deprivation impairs linear growth, this has widely been considered to be a function of indirect nutrient effects through modulation of the GH/IGF-I axis. However, our preliminary data indicate that the essential amino acid leucine can directly modulate chondrogenesis. Given that chondrocyte differentiation involves cellular hypertrophy, and that the recently characterized nutrient sensing pathway(mTOR) pathway regulates cell size in response to nutrient availability, we hypothesized that nutrient signaling via mTOR might exert direct and indirect effects on chondrocytes. We reported that direct mTOR inhibition with rapamycin, blocked differentiation of ATDC5 chondrogenic cells. Leucine restriction also inhibited differentiation and proliferation. Examination of the effects of rapamycin versus leucine restriction indicated significant difference in gene profile expression. Similar results in an intact in vivo system, fetal rat metatarsal explants, showed differences between the effects of rapamycin and leucine restriction. We further demonstrated that mTOR inhibition decreases Indian Hedgehog expression, which may account for decreased chondrocyte differentiation. This work has the potential to establish a role for nutrients in the indirect regulation of chondrogenesis via direct regulation via the mTOR pathway.